Ink-jet printheads employed in ink-jet plotters consume considerable quantities of ink. Such quantities require a means for storing sufficient ink for the useful life of the printhead. Further, ink must be supplied to the printhead under a prescribed negative pressure to prevent ink from dripping out of the nozzles. The word "ink" in this application means any liquid toner which is deposited on demand onto a recording medium.
An ink delivery system has been developed which is provided with a reservoir for supplying a refillable bladder. The bladder is then used to feed the printhead, and when the bladder is depleted, it is refilled from the reservoir, or ink bag. This ink delivery system is the subject of a separate patent, U.S. Pat. No. 4,714,937, issued on Dec. 22, 1987, and assigned to the assignee of the present application. In that system, a refillable bladder, a valve and an ink bag are utilized to deliver ink to the ink-jet printhead. The valve permits selective fluid communication between the ink bag and the bladder (refill mode) and between the bladder and the ink-jet printhead (print mode). A third position (shipping mode) prevents fluid communication between any of the components.
For an ink-jet printer incorporating a bladder, whether refillable, as above, or non-refillable, an indication of the quantity of ink remaining would give the user useful information about when to replace a disposable printhead or ink cartridge. Further, such an indication is important in determining when to actuate the valve to refill a refillable bladder from the reservoir.
Several means of sensing ink quantity and a low-ink condition are known in the art. These means include liquid level sensing in a fluid ink chamber by means of floats, optical probes, thermistors, conductivity sensors, and pressure probes. Capacitive sensing is sometimes used to determine the spacing between walls of a collapsing ink bag.
However, such prior art approaches suffer from a variety of deficiencies. Many are expensive per se, or require the addition of expensive components. Other prior art sensors are included as part of the disposable consumables. Still other sensors are intended to rely on the physical properties of inks, such as color or optical density, chemical composition, reactivity, mass density, viscosity or electrical conductivity, and are therefore limited in versatility.
Depending on a particular configuration, the prior art sensors may be subject to significant errors in determining the remaining ink level or reliably detecting a true out-of-ink condition if they do not actually sense ink quantity but rely upon indirect means, such as conductivity, capacitance, optical density, etc.
Thus, there remains a need to provide a means for sensing ink-quantity and low-ink condition in ink-jet printers which avoids most, if not all, of the foregoing limitations.